Optical fiber technology is currently applied in communication systems to transfer information, e.g. voice signals and data signals, over a long distance as optical signals. Over such long distance, however, the strength and quality of a transmitted optical signal diminish. Accordingly, techniques have been developed to regenerate or amplify optical signals as they propagate along an optical fiber.
One well known amplifying technique called Raman amplification exploits an effect called Raman scattering to amplify an incoming information bearing optical signal. Raman scattering describes the interaction of light with molecular vibration of the material through which the light propagates. And Raman amplification is based on a nonlinear effect of silica which is the main element of optical fibers. When exposed to a radiation, the material of the fiber absorbs a part of the energy (which corresponds to vibrational states of the molecular structure). Incident light scattered by molecules experiences a downshift in frequency from the power bearing optical pump radiation. This downshift in frequency (or increase in wavelength) from the pump radiation is referred to as the Stokes shift and the corresponding scattered light as Stokes-line. In fact, at least a second scattered light can be measured in a symmetric way respective to the optical pump radiation (therefore corresponding to an upshift in frequency) which is referred to as the anti-Stokes-line and considered usually as negligible. The extent of the shift and the shape of the Raman gain curve is determined by the molecular vibrational frequency modes of the transmission medium. In amorphous materials, such as silica, molecular vibrational frequencies spread into bands which overlap and provide a broadband wide gain curve. The efficiency or the characteristics of the Raman effect can be improved by introducing dopants in the fiber like Germanium or Phosphorus. An amplification based on the use of such amorphous material is usually called parametric amplification and is nowadays quite popular for amplifying optical signals partly due to its promise of noiseless signal regeneration.
A well promising amplification is based on the third order optical parametric effect obtained by coupling a signal wave (optical signal of carrier angular frequency ωs) with pump radiations (of carrier angular frequencies ωp1, ωp2) in an non-optical medium and propagating therefrom to induce the third order optical parametric effect. The optical signal is thus amplified and through a four-wave mixing process (shortened to FWM hereinbelow), a new radiation having a carrier angular frequency ωf is generated. Here, the carrier angular frequencies ωs, ωp1, ωp2, ωf of the optical signal, the pump radiation and the FWM generated radiation are governed by the law of conservation of energy as expressed in the following equation:ωs+ωf=ωp1+ωp2.
The generated FWM radiation has a mirror symmetry with the spectrum of optical radiation with respect to the carrier angular frequency (ωp1+ωp2)/2, and functions also as the optical phase conjugation radiation for the optical signal. When an optical circuit comprising such a non-linear optical medium is to be used as an FWM radiation generator, it is necessary to pack the carrier angular frequencies ωs, ωp1 and ωp2 to increase the conversion gain (expressed as FWM radiation intensity/optical signal intensity) of optical signal to FWM radiation, as well as increase the pump radiation intensity. Similarly, when such optical circuit is to be used as an optical parametric amplifier (shortened to OPA hereinbelow), it is necessary to pack the carrier angular frequencies ωs, ωp1, ωp2 and increase the pump radiation intensity to increase the amplification gain of the optical signal. When the requirement is to amplify optical signals then only latter configuration will be used. In that case, only the amplified optical signals at carrier frequency ωs will have to be collected at the output of the optical circuit. All the other radiations i.e. pump radiations ωp1, ωp2 and the generated FWM radiation ωf will have somehow to be filtered out to avoid any cross-talk.
It should he noted that, in addition to the generated FWM radiation of a carrier radiation frequency ωp1+ωp2−ωs(above defined as ωf) generated in the non-linear optical medium, unwanted FWM radiations of carrier frequencies 2ωp1−ωs, 2ωp2−ωs are produced by the degenerated pump radiations at a spacing |ωp1−ωp2| for different optical signals ωs. For optical signals of different wavelengths as used in wavelength division multiplexed WDM optical signals, if their carrier frequencies and the pump radiation satisfy the expression ωp1<ωsj<ωp2(j=1,2, . . . N) the FWM radiation Fj of carrier frequency ωp1+ωp2−ωsj excited by the non-degenerate pump radiations is generated between the frequencies ωp1, ωp2. And the unwanted FWM radiations also called idlers excited by the degenerated pump radiations of carrier frequencies 2ωp1−ωsj, 2ωp2−ωsj are generated in a range outside the frequencies ωp1, ωp2 (within “secondary” amplification bands). For OPA, not only the FWM radiation ωfj but also the idlers will have to be filtered out to collect only the amplified optical signals at carrier frequencies ωsj.
There is a limiting case of OPA which is of interest for the amplification of optical signals and corresponds to the case of the use of a single pump radiation. This limiting case, called degenerated OPA, is simply obtained when setting the two pump radiations of carrier frequencies ωp1, ωp2 to be equal. The previously described idlers obtained at 2ωp-s will coincide with the main FWM radiation F of the carrier frequency ωf the limiting case of a single used pump radiation of carrier frequency ωp. In fact, the optical signal of carrier frequency ωs and the FWM generated radiation of carrier frequency ωf correspond respectively to the Stokes line and to the anti-Stokes line symmetric between each other respective to the pump radiation of carrier frequency ωp. It must be noted that in the degenerated OPA the generated FWM radiation corresponding to the anti-Stokes line is of an order of the amplified optical signal and is therefore no longer negligible. In fact, it is very important to filter out such generated FWM radiation when collecting the amplified optical signals to avoid any cross-talk or mismatch. The two amplification bands (principle amplification bands) defined in the two pomp OPA in the intervals [ωp1, (ωp1+ωp2)/2] and [(ωp1+ωp2)/2, ωp2] if ωωp1<ωp2 will now be located on she right and the left of the used pump radiation of carrier frequency ωp.
In “Interleaver-Based Method for Full Utilization of the Bandwidth of Fiber Optical Parametric Amplifiers and Wavelength Converters” from M. Marhic et al., ThK4, OFC 2003, is presented a method using two parallel fiber optical parametric amplifiers (OPAs) and two interleavers. Such system is used to either amplify or spectrally invert a broad spectrum. Indeed, fiber OPAs can exhibit gain bandwidth of several hundred nanometers. However, if a densely populated wavelength division multiplexed WDM spectrum is presented at the input, covering the entire OPA bandwidth, then at the output the signal and idler spectra overlap completely, and it is not possible to place the idlers in gaps between the signals. Without modification, this fundamental problem limits the usable width of a WDM spectrum to about half the potential full OPA bandwidth. To use the full OPA bandwidth, one needs to use filters to separate the signals into two groups, and amplify them separately.
The method presented in the above paper ThK4 at OFC 2003 is based on the use of an interleaver, to separate even and odd channels. These are then amplified separately, and recombined by another interleaver. With a four-port second interleaver, the signal spectrum and the idler spectrum are simultaneously available from the two output ports. The method requires that the carriers and the interleavers be precisely aligned with a common ITU grid. Therefore, once the architecture is defined and the interleaver chosen, the channel spacing cannot be changed, and the system cannot be upgraded. Moreover, the number of channels is fixed, which is a very serious drawback of such solution.